1. Field of the Invention
The present invention relates to an optical glass, a press-molding preform, a process for producing the preform, an optical element and a process for producing the optical element. More specifically, the present invention relates to an optical glass having specific optical constants and having excellent climate resistance, particularly, an optical glass having a low-temperature softening property suitable for press-molding, a press-molding preform made of the above optical glass, a process for producing the preform, an optical element and a process for producing the optical element.
2. Prior Art
Optical glasses having optical constants such as a refractive index (nd) of 1.57 to 1.67 and an Abbe's number (νd) of 55 to 65 are useful as a material for optical elements such as a lens, and “Glass Data Book” issued by Japan Glass Product Society describes such glasses as SK type glass. However, these glasses are not suitable as a glass for precision press-molding, since they generally have a high glass transition temperature of over 560° C. When a glass for press-molding has a high molding temperature, there is caused a problem that the surface of a press mold may be damaged or that the material constituting the mold is degraded in durability, so that it is desirable that the glass transition temperature of the glass should be as low as possible. For overcoming the above problems, there is known a glass containing a larger amount of an alkali such as Li2O such as an SiO2—B2O3—SrO—Li2O glass (JP-B-2872899) or a glass containing an alkali and TeO2 such as an SiO2—B2O3—BaO glass (JP-B-3150992).
Meanwhile, the present inventors have noted that glasses having a refractive index (nd) of approximately 1.57 to 1.67 and an Abbe's number (νd) of approximately 55 to 65 generally have insufficient climate resistance. The climate resistance of a material glass is a very important property for using an optical element such as a lens in a good condition for a long period of time.
However, the glass disclosed in the above JP-B-2872899 contains large amounts of SrO and Li2O, so that it has defects that it is poor in climate resistance and that a press-molded product is liable to be cloudy when it is press-molded. Further, the glass disclosed in JP-B-3150992 seeks to decrease its glass transition temperature by introducing an alkali and TeO2. However, the glass is degraded in chemical durability to a great extent, so that it is unsuitable as a glass for press-molding (precision press-molding).
Under the circumstances, the present inventors have found it greatly useful for industrial developments in the field of this art to provide an optical glass having the above optical constants and a low-temperature softening property to permit press-molding and having excellent climate resistance.